Viewfinder optical system

ABSTRACT

A viewfinder optical system comprising a semi-transparent mirror disposed backwardly and obliquely of an image forming lens, an image forming plane and a directional or reflextive screen disposed at conjugated positions with respect to said semitransparent mirror and an optical system for viewing through said semi-transparent mirror an image formed upon said directional or reflexive screen.

United States Patent Ataka {54] VIEWFINDER OPTICAL SYSTEM [72] Inventor:Hisanori Ataka, Kawasaki, Japan [73] Assignee: Kabushiki Kaisha Ricoh[22] Filed: Feb. 26, 1970 [21] App1.No.: 14,289

[30] Foreign Application Priority Data Feb. 28, 1969 Japan ..44/15572[52] US. Cl ..95/42 [51] Int. Cl. ...G03b 19/12 [58] Field of Search.;.95/42 [56] References Cited UNITED STATES PATENTS 3,127,809 4/1964Denk ..95/42 X 2,323,005 6/1943 Bertele ..95/42X [151 3,656,421 [4 51Apr. 18,1972

2,252,640 8/1941 Nuchterlein ..95/42 3,468,233 9/1969 Schmidt ..95/423,103,150 9/1963 Lange FOREIGN PATENTS OR APPLICATIONS 508,396 1/1955Italy ..95/42 Primary ExaminerJohn M. Horan Assistant Examiner-MichaelHarris An0rneyBurgess, Ryan & Wayne [5 7] ABSTRACT A viewfinder opticalsystem comprising a semi-transparent mirror disposed backwardly andobliquely of an image forming lens, an image forming plane and adirectional or reflextive screen disposed at conjugated positions withrespect tosaid semi-transparent mirror and an optical system for viewingthrough said semi-transparent mirror an image formed upon saiddirectional or reflexive screen.

5 Claims, 4 Drawing Figures 1 VIEWFINDER OPTICAL SYSTEM BACKGROUND OFTHE INVENTION The present invention relates to an optical system of aviewfinder and more particularly a viewfinder optical system for camerasand projectors.

In order to view an erect image of an image formed upon a ground glassor screen, the prior art generally uses a pentaroof type prism or acombination of a plurality of reflecting mirrors. Therefore, theviewfinder optical system of the prior art is complicated inconstruction and very expensive.

SUMMARY OF THE INVENTION The present invention provides a viewfinderoptical system in which a sensitized member and a directional orreflexive screen are disposed at conjugated positions with respect to asemi-transparent mirror which in turn is disposed backwardly of a photolens and obliquely of the optical axis thereof so that an image formedupon the directional or reflexive screen may be viewed through thesemi-transparent mirror.

The novel feature which is characteristic of the present invention isthat the directional or reflexive screen is used as a ground glass andan image formed upon the directional or reflexive screen is viewedthrough an eyepiece as an enlarged, erect real image.

According to one embodiment of the present invention, the viewfinderoptical system is so arranged that an image of a subject may be viewedthrough an eyepiece in the direction parallel with the optical axis ofthe photo lens or at eye level while in another embodiment of thepresent invention the viewfinder optical system is so arranged that theimage of a subject may be viewed at a right angle relative to theoptical axis of the photo lens or at waist level.

Many advantages may be accrued from the present invention as will bedescribed below. The light reflected by the ground glass screen isdirectional or reflexive so that an image of a relatively higherluminance may be viewed even though a semi-transparent mirror is used. Areal image forming lens disposed on the side of the semi-transparentmirror opposite to the directional or reflexive screen may have aneffective diameter smaller than that of the photo lens so that thepresent invention encourages much freedom in camera design. In theconventional viewfinder system, a penta-roof type mirror or acombination of a plurality of reflecting mirrors is used in order toform an erect image of the image formed upon the ground glass, but inaccordance with the present invention, only two reflecting mirrors ofrelatively small size are used. Furthermore, one of the two reflectingmirrors may be so arranged as to be retracted from the optical system sothat the viewfinder system may be readily converted into a waist-levelviewfinder. A single-lens reflex camera to which the present inventionis applied can be made more compact in size and light in weight ascompared with the conventional camera.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a sectional view illustratingonly the optical system of a single-lens reflex camera to which thepresent invention is applied in one form;

FIG. 2 is a sectional view of another embodiment of the presentinvention illustrating only major components of a viewfinder;

FIG. 3 is a sectional view of a third embodiment of the presentinvention; and

FIG. 4 is a fragmentary sectional view, on enlarged scale, of adirectional or reflexive screen which is used as a ground glass inaccordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will bedescribed as being applied to a single-lens reflex camera.

As best shown in FIG. 4, a directional or reflexive screen 1 consists ofa multiplicity of small glass beads 2 which have a suitable refractiveindex and are overlaid upon a light reflecting material base 3 in such amanner that glass beads 2 may not be overlaid one upon another. Thelight incident upon the directional or reflexive screen is reflectedupon the spherical surface of each of the glass beads in contact withthe base 3 so that the reflected light is directed in substantially thesame direction in which the light is incident upon the screen.Therefore, an image having a relatively higher luminance may be observedfrom a relatively confined area adjacent to the light source from whichthe light is projected upon the screen.

Referring to FIG. 1, reference numeral 4 designates a photo lens of asingle-lens reflex camera whose outer configuration is indicated by thechain line 5; and 6, a semi-transparent mirror disposed upon theinterface between two right-angle prisms 7 and 8. The directional orreflexive screen 1 and a sensitized member 9 are placed at conjugatedpositions with respect to the semi-transparent mirror 6 so that theimages are formed by the photo lens 4 upon the screen 1 and upon thesensitized member 9. A real image forming lens 10 having a suitablefocal length is disposed on one side of the semi-transparent mirror 6opposite to the screen 1 and a field lens 11 is positioned at the focalpoint of the real image forming lens 10. An image formed by the lens 10can be viewed as an erect image through an eyepiece 14 and two flat orplanar mirrors l2 and 13 which are so arranged as to orient the opticalaxis of the lens 10 to the eyepiece.

When the mirror 12 is removed from the optical system and when theeyepiece is positioned upwardly of the field lens 11 the real image maybe viewed from the upper side of the camera. This arrangement isillustrated in FIG. 2 in which the mirror 12 is securely fixed to aframe 16 which in turn is rotatably fixed to a stationary member bymeans of a shaft 15 extending along the front side edge of the mirror12. The frame 16 may be rotated by an operation from the exterior of thecamera from the reflecting position indicated by the solid line to anon-reflecting position indicated by the broken line 16A. When themirror 12 is placed in the non-reflecting position 16A, the erect imageof a subject focused upon the field lens 11 may be viewed through aneyepiece 17.

FIG. 3 illustrates another embodiment of the present invention in whichthe field lens 11 is eliminated and a real image forming lens 27 ispositioned forwardly of the eyepiece 14. This arrangement has oneadvantage that the camera can be made compact in size especially in thedirection of its height. A bioconcave lens 28 positioned in front of theeyepiece 14 in the optical system shown in FIG. 1 or 2 serves to reducethe diameter of the light bundle between the field lens 11 and theeyepiece 14 so that the mirrors l2 and 13 interposed between the fieldlens 11 and the eyepiece 14 may be reduced in size.

The present invention has been so far described in detail as beingapplied to the single-lens reflex cameras, but it will be readilyunderstood that an optical projection system may be provided when thesensitized member 9 in FIG. 1 is eliminated while an image formed thereis projected upon a suitable screen.

lclaim:

l. A viewfinder optical system for a camera comprising a photo lenslocated on the optical axis of the camera;

a pair of right-angled prisms located on the optical axis adjacent saidphoto lens, said prisms having an abutting interface disposed obliquelyto the optical axis;

a semi-transparent mirror disposed on said interface;

a reflexive screen located in a plane spaced from a side of one of saidprisms;

a sensitized member located in a plane spaced from a side of the otherprism;

the plane of said member being generally perpendicular to the opticalaxis passing through said photo lens, and the plane of said screen beinggenerally parallel to the optical axis passing through said photo lens;

said member and said screen each forming an angle with respect to saidoblique interface, and said member and said screen being located atconjugated positions with 3. A viewfinder according to claim 1 in whichat least one of said mirrors is mounted to be pivoted out of the opticalaxis thereby allowing the image to be directed from said prisms forwaist-level viewing.

4. A viewfinder according to claim 1 in which said real image forminglens is positioned between said reflecting mirrors and said eyepiece.

5. A viewfinder according to claim 4 in which a biconcave lens islocated between said reflecting mirrors and said eyepiece.

1. A viewfinder optIcal system for a camera comprising a photo lenslocated on the optical axis of the camera; a pair of right-angled prismslocated on the optical axis adjacent said photo lens, said prisms havingan abutting interface disposed obliquely to the optical axis; asemi-transparent mirror disposed on said interface; a reflexive screenlocated in a plane spaced from a side of one of said prisms; asensitized member located in a plane spaced from a side of the otherprism; the plane of said member being generally perpendicular to theoptical axis passing through said photo lens, and the plane of saidscreen being generally parallel to the optical axis passing through saidphoto lens; said member and said screen each forming an angle withrespect to said oblique interface, and said member and said screen beinglocated at conjugated positions with respect to said oblique interfaceso that said angles are equal; an eyepiece located on the optical axison the side of said prisms opposite to the location of said screen; anda real image forming lens and a plurality of reflecting mirrors locatedon the optical axis between said prisms and said eyepiece.
 2. Aviewfinder according to claim 1 in which said real image forming lens isdisposed adjacent said prisms, and said reflecting mirrors are disposedadjacent said eyepiece to direct the image for eye-level viewing.
 3. Aviewfinder according to claim 1 in which at least one of said mirrors ismounted to be pivoted out of the optical axis thereby allowing the imageto be directed from said prisms for waist-level viewing.
 4. A viewfinderaccording to claim 1 in which said real image forming lens is positionedbetween said reflecting mirrors and said eyepiece.
 5. A viewfinderaccording to claim 4 in which a biconcave lens is located between saidreflecting mirrors and said eyepiece.